Montreal, October 2, 2006 -- A new study at the Université de Montréal has concluded that people drinking coffee to get through a night shift or a night of studying will strongly hurt their recovery sleep the next day. The study published in the current issue of Neuropsychopharmacology was conducted by Dr. Julie Carrier from the Department of Psychology at the Université de Montréal. Dr. Carrier runs the Chronobiology Laboratory at the Hôpital du Sacré-Coeur de Montréal.

"We already knew that caffeine has important effects on nocturnal sleep. It increases the time taken to fall asleep, it increases the amount of awakenings, and it decreases the amount of deep sleep. We have shown that these effects of caffeine on sleep are way stronger when taken at night prior to a daytime recovery sleep episode than in the evening before a nocturnal sleep episode."

"Caffeine makes daytime sleep episodes too shallow to override the signal from the biological clock that tells the body it should be awake at this time of day," explains Dr. Carrier. "We often use coffee and other sources of caffeine during the nighttime to counteract sleepiness generated by sleep deprivation, jet lag, and shift-work. However, this habit may have important effects when you then try to recuperate during daytime."

Thirty-four moderate caffeine consumers participated in both caffeine (200 mg) and placebo (lactose) conditions in a double-blind crossover design. Seventeen subjects followed their habitual sleep–wake cycle and slept in the laboratory during the night (Night), while 17 subjects were sleep deprived for one night and recovery sleep started in the morning (DayRec). All subjects received a capsule of 100 mg of caffeine (or placebo) 3 hours before bedtime, and the remaining dose 1 hour before bedtime. Compared to placebo, caffeine lengthened sleep latency, increased stage 1, and reduced stage 2 and slow-wave sleep (SWS) in both groups. However, caffeine reduced sleep efficiency more strongly in the DayRec group, and decreased sleep duration and REM sleep only in that group.

People who stay up all night feel the greatest need to use coffee and other sleep-fighting stimulants. But they should most try to avoid use of coffee since they need quality deep sleep when they try to sleep during the day. Sorry about that late night workers.